fix: document.add does not work on Linux (#487)

Co-authored-by: Rijken, M.A.M.M. (Marc) <[email protected]>

Author: mrijken

src/nitpick/blender.py

@@ -551,7 +551,7 @@ def traverse_toml_tree(document: tomlkit.TOMLDocument, dictionary):
             if key in document:
                 traverse_toml_tree(document[key], value)
             else:
-                document.add(key, value)
+                document[key] = value
         else:
             document[key] = value